1
general::
Debunk, render useless or unbelievable. For example, His findings knocked our theory into a cocked hat. This expression alludes to a style of hat with the brim turned up on three sides? the three-cornered (tricorne) hat worn by officers in the American Revolution? giving it a distorted look. [Early 1800s]
American Heritage Idioms
2
general::
Phrase(s): knock something into a cocked hat
Fig. to demolish a plan, a story, etc. • I knocked his plans into a cocked hat. • This bad weather has knocked everything into a cocked hat.
McGrawhill's American Idioms And Phrasal Verbs